Disk drive devices using various kinds of disks, such as optical disks, magneto-optical disks, flexible magnetic disks, and the like have been known in the art. In particular, hard disk drives (HDDs) have been widely used as storage devices of computers and have been one of the indispensable storage devices for current computer systems. Moreover, HDDs have found widespread application to moving image recording/reproducing apparatuses, car navigation systems, cellular phones, and the like, in addition to the computers, due to their outstanding characteristics.
Magnetic disks used in HDDs have multiple concentric data tracks and servo tracks. Each servo track is constituted by a plurality of servo data containing address information. Each data track includes multiple data sectors containing user data recorded thereon. Data sectors are recorded between servo data located discretely in the circumferential direction. A head element portion of a head slider supported by a swing actuator accesses a desired data sector in accordance with address information in the servo data to write data to and retrieve data from the data sector.
To increase recording density of a magnetic disk, it is important to decrease the clearance between a head element portion flying over a magnetic disk and the magnetic disk. Some mechanisms have been proposed to control the clearance. One of such mechanisms has a heater in a head slider; the heater heats the head element portion to adjust the clearance. In the present specification, this mechanism is referenced as thermal fly-height control (TFC). The TFC generates heat by applying electric current to the heater to make the head element portion protrude by thermal expansion. This reduces the clearance between the magnetic disk and the head element portion.
The clearance varies with barometric pressure (altitude) as well as temperature. If the clearance preset value in a read/write operation is 5 nm or more, the clearance variation caused by an altitude change can be absorbed by a clearance margin. However, if the clearance is not more than 2 or 3 mm in a read/write operation, clearance control for a pressure change in addition to a temperature change is demanded.
Typical TFC makes a head element portion protrude due to thermal expansion by increasing heater power in response to a decrease in temperature to compensate for the increase in clearance caused by the decrease in temperature. In contrast, as the altitude gets higher and the barometric pressure (hereinafter, referred to as pressure) becomes lower, the fly-height of a slider lowers. The lowered pressure reduces the clearance between the head element portion and the magnetic disk. Therefore, if the temperature is constant, the TFC decreases the protruding amount with decrease in pressure.
An HDD has a number of preset parameters for temperature; accurate temperature sensing is indispensable for normal operation of the HDD. Therefore, a common HDD comprises a temperature sensor as a means to sense the temperature. A barometric sensor (altitude sensor) has been known as a means for sensing the pressure. However, use of a barometric sensor increases the number of components in the HDD and the cost of the HDD as well significantly. Since there are few parameters to be set for pressure change except for the parameters for clearance control, it is preferable to determine the pressure without a barometric sensor.
As described above, the clearance varies with pressure. Accordingly, a variation in pressure can be measured by referring to the clearance. Some techniques to determine a clearance have been known. A typical technique determines a clearance (a variation in clearance) from the amplitudes of read signals of a head element portion. One of the methods for determining a precise clearance using the amplitude of read signals determines the clearance from the resolution of frequency components having different amplitudes of read signals.
For clearance control depending on pressure without a barometric sensor, it is demanded to determine clearance variation by referring to read signals of the head slider and then to determine pressure variation from the clearance variation. The clearance, however, varies with temperature. Accordingly, to determine a pressure variation from a clearance variation, it is demanded to compensate for the component of the clearance variation caused by a temperature change.
As described above, the clearance can be precisely measured from the resolution. Among some existing method to measure the resolution, a method has been known to determine the resolution from values used in digital signal processing in a read-write channel (RW channel) (for example, refer to Japanese Patent Publication No. H5-81807 “Patent Document 1”). A controller of an HDD obtains the above-mentioned digital values from the RW channel and determines the resolution from the values.
As described above, the resolution varies depending on clearance which varies with temperature. Specifically, as the clearance decreases, the resolution increases. However, it has been found that, when the resolution is measured by using values in the RW channel as described above, the measured resolution varies with temperature due to factors other than the clearance variation. Thus, the operation in the RW channel is affected by temperature so that the measured resolution is varied by the change in the operational property.
FIG. 10 shows the measured data indicating the relationship between the specific value representing the resolution in the RW channel (the value referenced as Kgrad below) and the temperature in the HDD. Setting the specific value at 30° C. for the default, the temperature in the HDD and the specific value were measured. The X-axis represents temperature, and the Y-axis represents variation in the specific value. The specific value represents the resolution and is used in the digital signal processing in the RW channel. Since the clearance decreases with rising in temperature, it is assumed that the resolution should increase with rising in temperature. However, the measured data shown in FIG. 10 do not show such change. The variation in the measured value caused by operational property change of the RW channel has balanced out the variation in the measured value caused by clearance change.
Studying the circuit configuration in the RW channel, it has been found that a waveform equalization filter (low-pass filter) particularly shows remarkable property change depending on temperature, which significantly affects the measurement of resolution. The waveform equalization filter is a pre-step circuit of the AD converter in the RW channel and has a gain property depending on frequency. The gain property changes with the temperature of the RW channel. FIG. 11 shows change in the gain of the waveform equalization filter depending on temperature; the X-axis represents frequency, the Y-axis represents gain, and each graph represents the measured values at a different temperature. As shown in FIG. 11, the gain of the filter decreases at all of the frequencies as the temperature rises, and the decrease in gain at higher frequencies is greater.
Resolution can be expressed by the ratio between the high frequency components and low frequency components of signals. Accordingly, the measured resolution is decreased by the above-described property change of the low-pass filter caused by rising in temperature. On the other hand, the clearance decreases with rising in temperature and the measured resolution caused thereby increases. Accordingly, the variation in the measured resolution is smaller, comparing with the clearance variation by temperature change. For all of the above reasons, to measure the resolution using the value in the RW channel and precisely measure the clearance variation from the resolution, it is demanded to correctly compensate for the variation in the measured resolution which is caused by property change of the RW channel depending on temperature.